The present invention relates generally to an optical system driving apparatus for driving a driven unit including an objective lens in a direction corresponding to the optical axis of the objective as well as in a direction perpendicular to the optical axis, for optically recording and/or reading information or data from a recording medium with the aid of a light beam transmitted through the objective.
In previously proposed optical recording/reproducing apparatus using a laser light beam, the laser light beam is focussed in a minute spot by an objective lens system for the signal recording and/or reading (detecting). For accomplishing the signal recording and/or detection (reading) with desired accuracy, it is necessary not only to provide a focussing control for forming the light spot focussed on an information recording medium in consideration of roughness or physical non-uniformity and vibration of the information recording medium but also a tracking control for causing the focussed light spot to constantly trace the correct signal track without fail. Further, to deal with errors brought about in the time base due to non-unformity in rotation of the information recording carrier, it is necessary to provide an error detection system combined with means for controlling an actuator adapted to drive the objective lens system so that the error can be cancelled.
Previously for the error correcting control mentioned above, there has been known a driving mechanism in which the objective lens is displaced in the direction of the optical axis thereof while mirrors located on the optical axis leading to the objective and having respective rotation axes extending orthogonally to each other are rotated by using a so-called voice coil, to thereby cause the optical path to be shifted in two directions perpendicular to the optical axis, whereby position of the focussed spot of laser light beam is controlled in the three independent directions. The driving system of this structure requires a heavy objective lens having a large angle of view, resulting in that the overall size of the driven unit is remarkably increased, to a disadvantage.
As an approach to solve the above problem, there has been proposed an arrangement in which the optical system including a light-weight objective having a small angle of view is supported by means of resilient supporting members such as leaf springs so that the optical system can be vibrated in the direction corresponding to the optical axis and the direction perpendicular thereto by utilizing electrical signals. For driving electrically the objective lens two-dimensionally, it is possible to adopt a driving system in which electromagnetic means is employed, a driving system implemented by resorting to the use of the voice-coil means and a driving system in which piezo-electric elements are used. In any system as adapted, it is necessary to implement the driven unit in a reduced size and weight in order to assure an improved response in the focussing and tracking performances. In this connection, it has been proposed to provide a structure known heretofore in which a driven unit is equipped with a coil while a magnetic circuit is employed for the driving unit for implementing the tracking function as well as the focussing function. A disadvantage of this proposed structure resides in the fact that manufacturing cost are increased because of severe requirement imposed on the dimensional accuracy of the coil. Further, this proposed structure is not suited for mass-production fabrication. Besides, in case the structural rigidity or stiffness of the coil assembly is low, it is very difficult to increase the upper resonant frequency (f.sub.h) of the driven unit inclusive of the coil beyond the frequency band of a servo system, giving rise to a problem that the focussing and tracking operations can not be carried out with a desired accuracy.